A spectroscopic sensor is used for non-invasive examination in various fields such as medical treatment, cosmetic care, and health.
A general spectroscopic sensor irradiates a subject with electromagnetic light from a visible light source, an infrared light source or the like, or by a laser, an LED or the like that radiates light at narrow band wavelength, and causes the reflected light or a light component shifted by Raman scattering to pass through a slit to be transmitted through/reflected by a grating. The spectroscopic sensor thereby converts a signal intensity distribution in the wavelength direction into a spatial signal intensity distribution.
Then, an incident spectrum may be restored by detecting the electromagnetic wave intensity of each wavelength component which has been spatially separated by a one-dimensional linear sensor or a two-dimensional sensor.
Now, as a solid-state image sensor to be used as a detector, there may be cited a CCD (Charge Coupled Device)-type solid-state image sensor or a CMOS (Complementary Metal Oxide Semiconductor)-type solid-state image sensor.
These image sensors are basically equivalent to the image sensor used in a general portable information terminal such as a digital still camera, video camera recorder or a smartphone, and the number of pixels of a solid-state image sensor for general imaging use is great, being more than ten million.
With both the solid-state image sensor used for the spectroscopic sensor and the solid-state image sensor used for general imaging use, each pixel of the image sensor accumulates signal charge according to the light intensity from the subject, and samples electrical signals that are according to the amount of charge accumulated as analogue or digital data and forms an image.
Now, regardless of being a spectroscopic sensor or an imaging sensor, the solid-state image sensor has sensitivity in a specific electromagnetic wavelength band.
For example, many of the CCD- or CMOS-type solid-state image sensors used in visible light/near-infrared light bands are fabricated based on silicon. Silicon has sensitivity only with respect to a wavelength shorter than that of a near-infrared ray (1.1 μm or less) due to its band gap.
However, there is no energy resolution (wavelength resolution) with respect to an electromagnetic wave with wavelength shorter than 1.1 μm, and it is difficult to identify light of which wavelength is detected based on the accumulated charge. Accordingly, the spectroscopic sensor generally uses a grating to enable detection of information of light intensity for each color/wavelength.
As an inevitable issue in the case of dispersing light by a grating, there is an issue of the energy of the light being spatially separated in the wavelength direction.
That is, to realize a spectroscope with high wavelength resolution (high dispersion) to dilute, in the wavelength direction, and detect the total light from a subject, the sensitivity of the solid-state image sensor has to be increased to that extent, or the integral time has to be increased.
Furthermore, there is also an issue that, since the incident light has to be transmitted through a narrow slit, the amount of light entering the sensor is small in the first place.
On the other hand, with a general color imaging device, the following method is adopted in many cases to acquire a color image.
That is, a method is often adopted according to which several types of on-chip color filters for selectively allowing a specific wavelength component to pass to each of two-dimensionally arranged pixels are provided, light intensity information for a plurality of wavelengths is acquired from a small number of adjacent pixel groups, and a color image is restored by an interpolation process by demosaicing.
In the case of the method above of arranging several types of filters on a two-dimensional pixel plane, unlike with the grating or the slit structure described above, light is not removed by a slit, but there is a big issue in the case of dispersion with high wavelength resolution.
That is, since organic materials such as dye and pigment forming the filters are formed by being applied, it is literally not possible to apply several types of filters at once.
That is, compared to a normal imaging device for synthesizing a color image from three colors, R, G and B, the cost is greatly increased for a spectroscopic device which requires a very large number of filters of ten colors or twenty colors, and realization is not easy.
However, in recent years, a hole array structure where holes of about the same degree or finer than a detection wavelength are periodically arranged on a conductive thin film, or an island array structure which is in negative and positive relation with the above structure is known as a plasmon resonator structure.
Moreover, the plasmon resonator structure is reported to function as a filter capable of adjusting the transmission wavelength by a physical structure when the periodicity and hole/dot shapes are optimized (see Non-Patent Documents 1 and 2).
Furthermore, techniques for using the plasmon resonator as a color filter are also disclosed (see Patent Documents 1, 2 and 3).
These techniques are advantageous in that, since each of the filters may be realized by patterning according to a periodic pattern on a metal thin film, many types of filters may be applied at once.